1. Field of the Invention
The present invention relates to a ceramic chip-type device having a glass coating film and a fabricating method thereof, and more particularly, to a chip-type varistor having a glass coating film and a fabricating method thereof, in which a coating film having an excellent acid-resistant property is formed on the surface of the ceramic chip-type device, to thus stand an attack due to a flux at the time of reflow soldering, to thereby maintain an initial insulation resistance.
2. Description of the Related Art
Recently, a variety of portable electronic equipment such as mobile telecommunication terminals becomes more compact in size. Accordingly, circuitry components used in such portable electronic equipment become also more compact and integrated in high density. As a result, each component is designed to have a low rating voltage and current.
In general, a varistor is a resistor element having a non-linear voltage/current characteristic. A large-capacity varistor for protecting a lightening arrester or a voltage transformer from an applied overvoltage has a structure in which a SiC fuse is inserted between both electrodes. Meanwhile, a compact small-capacity varistor which can react upon a relatively low voltage/current quickly has a structure in which a pair of conductor patterns connected to both electrodes are embedded in the ceramic material.
Meanwhile, when a chip-type varistor fabricated for use in a SMD (Surface Mounting Device) is mounted on a PCB (Printed Circuit Board) 3 using reflow soldering, both electrodes 9a and 9b of the chip varistor 1 contact solder pastes 5 and the bottom surface of the chip varistor 1 is eroded by a flux 7 as shown in FIG. 1A.
In general, a solder paste which is used for reflow soldering of a mounting chip component for a SMD uses a flux in order to enhance a soldering performance. The flux contains Clxe2x88x92 ion components, which play a role of removing foreign matters, dirts, oxides, and so on which exist on the device surface or external electrodes during soldering.
However, the flux component is activated in a reflow oven during soldering, and then a liquefied flux moves to between the PCB 3 and the chip varistor 1 as shown in FIG. 1B, to accordingly erode the surface of the chip varistor, particularly, a grain boundary 1a. Thus, the flux component attacks the surface of the chip varistor device during soldering, and dissolves ZnO and Sb2O3 having a low acid-resistant property among main constituents such as ZnO, Bi2O3, and Sb2O3. As a result, excessive Zn and Sb ions are made to exist in the flux.
The flux containing metal of the ionic phase forms another current flowing path between both the electrodes 9a and 9b in the chip varistor 1. Accordingly, after reflow soldering, an initial insulation resistance value of the chip varistor 1 falls down from several hundred Mxcfx89 through several Gxcfx89 to several hundred Kxcfx89 through several Mxcfx89 abruptly.
Further, in the case of a conventional chip varistor fabrication process, an external electrode terminal connected to an internal electrode terminal is formed and then the surface of the external electrode terminal is plated with metal such as Cu, Ni, and Sn.
Meanwhile, a general chip varistor is a product using a semiconductor property of a ZnO ceramic material, which plays a role of a non-conductor at normal state, but of a conductor at threshold voltage or higher. Thus, during electroplating of the chip varistor, the ceramic body is altered into a conductor and thus the surface of the ceramic body is plated. As a result, a bridging phenomenon that both external electrodes are connected each other may occur. Such a bridging phenomenon causes leakage of current to thereby provide a malfunction factor.
Further, since low voltage driving circuits are widely used recently, if an insulation resistance of a certain chip component falls down to a threshold value or less, an excessive current flows to thereby cause the circuit not to operate.
To solve the above problems, it is an object of the present invention to provide a chip-type varistor having a glass coating film preventing a bridging phenomenon during electroplating of external electrodes and a fabricating method, in which a coating film having an excellent acid-resistant property is formed on the surface of the ceramic chip device, to thus stand an attack due to a flux at the time of reflow soldering, to thereby maintain an initial insulation resistance.
It is another object of the present invention to provide a chip-type device fabricating method for forming a glass coating film on the surface of a general chip-type passive device and a ceramic chip device therefor in addition to the above chip varistor.
To accomplish the above object of the present invention, there is provided a chip-type varistor for maintaining an initial insulation resistance during soldering, the chip-type varistor comprising: a varistor chip in which a number of conductive pattern layers are stacked between the upper and lower portions in a ceramic body which are spaced by a predetermined distance, and whose both ends are withdrawn in either lateral direction in turn to thereby form first and second inner electrodes; a pair of first outer electrodes each surrounding either end of the varistor chip so as to be electrically connected to the first and second inner electrodes, respectively; and a glass coating film formed of an excellent acid-resistant material on the surface of the ceramic body in order to avoid erosion with respect to a grain boundary of the ceramic body surface due to a flux during soldering to thereby maintain the initial resistance.
The glass coating film can be extensively formed on the whole surface of the varistor chip. Also, the chip-type varistor further comprises a pair of second outer electrodes surrounding the pair of the first outer electrodes, respectively.
According to a first aspect of the present invention, there is also provided a method for fabricating a chip-type varistor having a glass coating film, the chip-type varistor fabrication method comprising the steps of: (a) preparing a varistor chip whose both ends are withdrawn in either lateral direction in turn to thereby form first and second inner electrodes, in which a number of conductive pattern layers are stacked between the upper and lower portions in a ceramic body which are spaced by a predetermined distance; (b) forming a pair of first outer electrodes each surrounding either end of the varistor chip so as to be electrically connected to the first and second inner electrodes, respectively; (c) forming a mask for preventing glass from being penetrated toward the inner electrodes in which polymer is used on the lower ends of the first outer electrodes; (d) after dipping the first outer electrodes into a glass-added paste, flowing the glass included in the paste onto the surface of the ceramic body by a thermal treatment to thereby form the glass coating film and simultaneously removing a face portion formed outside the mask so as to expose the first outer electrodes; and (e) forming a pair of second outer electrodes surrounding the pair of the first outer electrodes, respectively on either end of the chip.
According to a second aspect of the present invention, there is also provided a method for fabricating a chip-type varistor having a glass coating film, the chip-type varistor fabrication method comprising the steps of: (a) preparing a varistor chip in which a number of conductive pattern layers are stacked between the upper and lower portions in a ceramic body which are spaced by a predetermined distance, and whose both ends are withdrawn in either lateral direction in turn to there by form first and second inner electrodes; (b) dipping the varistor chip into a weak acid solution to thereby form a number of pores on the surface of the ceramic body; (c) after fully dipping the varistor chip into a glass slurry formed of glass powder, rotating and drying the chip so as to process the glass slurry coated on the surface of the chip to have a constant thickness; thermally treating the glass slurry coated chip to thereby melt the glass in the pores on the chip surface, and form a uniform glass coating film by a capillary phenomenon; and (d) forming respectively outer electrodes surrounding the glass coating film corresponding to the inner electrodes, on either end of the chip.
In this case, the glass-added paste is made of adding any one of SiO2+RO, B2O3+RO and SnO2+RO by 0.1-100 wt % to any one metal powder among Ag, Ag/Pt, Ag/Pd, Ag/Pd/Pt, Ag/Au and Ag/Au/Pt, in which RO is made of a mixture of one through five kinds of materials selected from the group consisting of PbO, Bi2O3, SiO2, Al2O3, ZnO, P2O5, MgO, Na2O, BaO, CaO, K2O, SrO, Li2O, TiO2, ZrO2, V2O5 and SnO2.
Also, it is preferable that the glass slurry comprises powders made of SiO2, Al2O3, CaO, Na2O, B2O3 and PbO, as a main component.
In this case, the outer electrode formation step comprises the steps of preliminarily forming the outer electrodes using a paste made of metal powder of 91-96 wt %, binder of 3 wt %, and glass of 1-5 wt %; and thermally treating the preliminary formed outer electrodes at 600-800xc2x0 C.
According to a third aspect of the present invention, there is also provided a method for fabricating a chip-type varistor having a glass coating film, the chip-type varistor fabrication method comprising the steps of: (a) pattern-printing an inner electrode formation conductive paste on a number of ceramic substrates to thereby prepare a number of inner electrode layers; (b) forming a pair of glass-added sheets in which glass is added to the same ceramic substrate as the above composition by 0.1-10 w %; (c) after collating, laminating and compressing the pair of glass-added sheets and undergoing a chip cutting in which the pair of glass-added sheets are used as upper and lower cover sheets for the inner electrode layers, sintering the glass components of the glass-added sheets in liquid phase in advance by performing burn-out and cofiring a binder, and then forming a glass coating film on a grain boundary of a ceramic body; and (d) after passing through a tumbling process, forming outer electrode terminals on either end of the chip.
Further, according to the present invention, there is also provided a ceramic chip-type device having a glass coating film, the ceramic chip-type device comprising: a ceramic passive chip including a pair of external electrode terminals on either end of the ceramic chip-type device; and a glass coating film of an excellent acid-resistant property formed on the surface of a ceramic body located between the pair of external electrode terminals.
As described above, glass having the excellent acid-resistant property is coated on the surface of the chip-type varistor in the present invention, to thereby prevent erosion of the chip-type varistor due to an activated liquified flux during reflow soldering. As a result, the present invention in which the glass coating film is formed can exclude an effect of the flux, to thereby maintain a high initial insulation resistance value.
Also, it is possible to remove a bridging phenomenon since the glass coating film protects the surface of the chip-type varistor from a plating solution during electrolytic plating.